Magnetic disk device

ABSTRACT

According to one embodiment, a magnetic disk device includes a magnetic disk, a recording head and a controller. The recording head includes a high-frequency oscillator disposed in a write gap between a main magnetic pole and a return magnetic pole and configured to oscillate in accordance with a bias voltage, and a bias voltage supply circuit configured to supply the bias voltage to the high-frequency oscillator. The controller includes a bias voltage controller configured to control the bias voltage to be applied to the high-frequency oscillator in accordance with a sampling frequency of data when the data is recorded on the magnetic disk by the recording head.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-in-Part Application of U.S. patentapplication Ser. No. 16/247,821, filed Jan. 15, 2019 and based upon andclaiming the benefit of priority from Japanese Patent Application No.2018-157373, filed Aug. 24, 2018, the entire contents of all of whichare incorporated herein by reference.

BACKGROUND Embodiments described herein relate generally to a magneticdisk device using a perpendicular magnetic recording head.

Recently, a perpendicular magnetic recording system has been adopted inmagnetic disk devices to increase their recording density and capacity.In the magnetic disk devices of this system, a recording head forperpendicular magnetic recording is opposed to the recording surface ofa magnetic disk having a recording layer for perpendicular magneticrecording. With the recording head, data is recorded in a predeterminedarea of the magnetic disk by producing a perpendicular-directionmagnetic field corresponding to the data to be recorded.

The recording head has a narrowed portion made of soft magnetism metaland includes a main magnetic pole that generates a magnetic field in theperpendicular direction, a return magnetic pole opposed to the mainmagnetic pole with a write gap therebetween to return magnetic flux fromthe main magnetic pole and form a magnetic circuit together with themain magnetic pole, and a coil that excites magnetic flux in themagnetic circuit formed by the main magnetic pole and the returnmagnetic pole to generate a recording magnetic field.

The recording head so configured also includes a high-frequencyoscillator, for example, a spin torque oscillator (STO) in the write gapto improve recording capability.

When the sampling frequency of recording data increases, the oscillationresponsiveness of the high-frequency oscillator becomes insufficient andthus the high-frequency oscillator cannot be brought into a stableoscillation state within a 1-bit time length of the data. Consequently,when data of high sampling frequency is recorded, the oscillation of thehigh-frequency oscillator becomes unstable to interfere in an adjacenttrack and cause the adjacent track to deteriorate in quality.

Embodiments described herein aim to provide a magnetic disk devicecapable of suppressing deterioration of signal quality of an adjacenttrack due to the oscillation responsiveness of a high-frequencyoscillator and improving the signal quality and the recording density.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram schematically showing a magnetic disk device(HDD) according to a first embodiment.

FIG. 2 is a partially enlarged sectional view schematically showing ahead portion of a magnetic head and a magnetic disk in the HDD accordingto the first embodiment.

FIG. 3 is a block diagram showing a system that controls SO bias voltageand polarity in accordance with a data sampling frequency in the firstembodiment.

FIG. 4 is a flowchart showing a process of controlling SIO bias voltageand polarity in accordance with a data sampling frequency in the firstembodiment.

FIG. 5 is a characteristic graph showing an example of setting an STObias voltage in accordance with a data sampling frequency in the firstembodiment.

FIGS. 6A and 6B are characteristic graphs showing their respective biasdependencies of delta R and delta OW in the first embodiment.

FIG. 7 is a characteristic graph showing data frequency dependency ofthe rate of improvement in recording density at the time of applicationof STO bias voltage in the first embodiment.

FIG. 8 is a waveform chart illustrating an effect of reducing an erasewidth by reversing recording current in the first embodiment.

FIG. 9 is a characteristic graph showing an effect of improving inrecording density in the first embodiment.

FIG. 10 is a flowchart showing a process of controlling STO bias voltageand polarity in accordance with the radial position of a write disk in asecond embodiment.

FIG. 11 is a characteristic graph showing an example of setting an STObias voltage corresponding to the radial position of the write disk inthe second embodiment.

FIG. 12 is a characteristic graph showing an example of setting thepolarity of an STO bias voltage corresponding to a data samplingfrequency in a third embodiment.

FIG. 13 is a characteristic graph showing an example of setting thepolarity of an STO bias voltage corresponding to the radial position ofa write disk in the third embodiment.

FIGS. 14A and 14B are characteristic graphs showing their respectiverecording current dependencies of bit error rate in a fourth embodiment.

FIG. 15 is a graph showing a result of measuring a Raman spectroscopicintensity ratio to observe temperature resistance of a DLC protectivefilm in a fifth embodiment.

FIG. 16 is a graph showing a result of calculating a rising temperatureof STO when applying the bias voltage to the STO in an environment of aconstant temperature in the device, in the fifth embodiment.

FIG. 17 is a graph showing an example of controlling the bias voltage inaccordance with a circumferential speed so as to prevent a temperaturenear the STO from exceeding a threshold value, in the fifth embodiment.

FIG. 18 a graph showing a situation of controlling the bias voltage tobe suppressed when a data sampling frequency is less than or equal to apredetermined threshold value, in the fifth embodiment.

FIG. 19 is a graph showing a situation of controlling the bias voltageto be suppressed in an area where a recording position of a disk isinner than a predetermined radial position, in the fifth embodiment.

FIG. 20 is a graph showing a situation of controlling the STO biasvoltage to be reduced, preventing deterioration of the DLC protectivefilm based on a write time and suppressing rise of the bit error rate,in the fifth embodiment.

FIG. 21 is a graph showing a result of calculating a temperature of theSTO when applying the bias voltage to the STO relative to a deviceinternal temperature at each portion, in an innermost circumferentialarea of a disk, in a sixth embodiment.

FIG. 22 is a graph showing an example of controlling the bias voltage inaccordance with a temperature inside the device so as to prevent atemperature near the STO from exceeding a threshold value of acompensated temperature.

DETAILED DESCRIPTION

Embodiments will he described hereinafter with reference to theaccompanying drawings.

In general, according to one embodiment, a magnetic disk devicecomprises a magnetic disk including a recording layer, a recording headconfigured to apply a recording magnetic field to the recording layer bya main magnetic pole and a return magnetic pole, and a controllerconfigured to control the recording head. The recording head includes ahigh-frequency oscillator disposed in a write gap between the mainmagnetic pole and the return magnetic pole and configured to oscillatein accordance with a bias voltage, and a bias voltage supply circuitconfigured to supply the bias voltage to the high-frequency oscillator.The controller includes a bias voltage controller configured to controlthe bias voltage to be applied to the high-frequency oscillator inaccordance with a sampling frequency of data when the data is recordedon the magnetic disk by the recording head.

What is disclosed in this specification is merely an example.Appropriate modifications which can be easily conceived by a personordinarily skilled in the art without departing from the spirit of theembodiments naturally fall within the scope of the present. invention.To further clarify explanation, for example, the width, thickness orshape of each structure may be schematically shown in the drawingscompared with the actual forms. Note that the drawings are merelyexamples and do not limit the interpretation of the present invention.In the specification and drawings, elements which are identical to thoseof the already-mentioned figures are denoted by the same referencenumbers. Thus, the detailed explanation of such elements may be omitted.

First Embodiment

The configuration of a magnetic disk device (a hard disk drive that willbe referred to as an HDD 10 hereinafter) according to a first embodimentwill be described with reference to FIGS. 1 and 2. FIG. 1 is a blockdiagram schematically showing the HDD 10. FIG. 2 is a partially enlargedsectional view schematically showing a head portion of a magnetic headand a magnetic disk. FIG. 3 is a block diagram showing a control systemthat controls a bras voltage (referred to as an STO bias voltagehereinafter) to drive an STO.

As shown in FIG. I, the HDD 10 includes a rectangular casing 11, amagnetic disk 12 provided in the casing 11 as a recording medium, aspindle motor 14 that supports and rotates the magnetic disk 12, and aplurality of magnetic heads 16 that writes and reads data to and fromthe magnetic disk 12. The HDD 10 also includes a head actuator 18 thatmoves the magnetic heads 16 onto a given track on the magnetic disk 12and positions them thereon. The head actuator 18 includes a suspensionassembly 20 that movably supports the magnetic heads 16 and a voice coilmotor (VCM) 22 that rotates the suspension assembly 20.

The HDD 10 includes a head amplifier IC 30, a main controller 40 and adriver IC 48. The head amplifier IC 30 is, for example, provided in thesuspension assembly 20 and electrically connected to the magnetic heads16. The main controller 40 and driver IC 48 are formed on, for example,a control circuit board (not shown) provided on the back of the casing11. The main controller 40 includes an R/W channel 42, a hard diskcontroller (HDC) 44 and a microprocessor (MPU) 46. The main controller40 is electrically connected to the head amplifier IC 30 and alsoelectrically connected to the VCM 22 and the spindle motor 14 via thedriver IC 48. The HDD 10 can be connected to a host computer (notshown).

The magnetic disk 12 is a perpendicular magnetic recording medium with arecording layer having anisotropy in a direction perpendicular to thesurface of the magnetic disk. Specifically, the magnetic disk 12includes a substrate 101 that is formed of a nonmagnetic body shapedlike a circular disc whose diameter is, for example, about 2.5 inches(6.35 cm).

The suspension assembly 20 includes a bearing section 24 rotatably fixedto the casing 11 and a plurality of suspensions 26 extending from thebearing section 24. The magnetic heads 16 are supported at theirrespective extending ends of the suspensions 26. The magnetic heads 16are electrically connected to the head amplifier IC 30 via aninterconnect member provided in the suspension assembly 20.

The configuration of the magnetic heads 16 will be described in detailbelow.

As shown in FIG. 2, each of the magnetic heads 16 is configured as aflying head and includes a slider 15 shaped almost like a rectangularparallelepiped and a head section 17 formed at an outflow end (trailingend) of the slider 15. The slider 15 is formed of, for example, asintered body of alumina and titanium carbide (AlTiC) and the headsection 17 is formed by a plurality of thin films.

The slider 15 includes a rectangular air bearing surface (ABS) 13opposed to the surface of the magnetic disk 12. The slider 15 remainsfloated by a predetermined amount from the surface of the magnetic disk12 by an air flow produced between the surface of the magnetic disk 12and the ABS 43 by the rotation of the magnetic disk 12. The direction ofthe airflow coincides with the direction of rotation of the magneticdisk 12. The slider 15 includes a leading end 15 a located on the inflowside of the air flow and a trailing end 15 b located on the outflow sideof the air flow.

The head section 17 is a separate magnetic head including a reproductionhead 54 and a recording head 58 formed at the trailing end 15 b of theslider 15 by a thin film process. In order to control therecording/reproduction floating amount, of the head section 17, arecording heater 19 a is disposed on the depth side of the recordinghead 58 and a reproduction heater 19 b is disposed on the depth side ofthe reproduction head 54.

The reproduction head 54 is configured by a reproduction element 55 of amagnetic film that exhibits a magnetoresistive effect and upper andlower shields 56 and 57 of shield films arranged at the trailing andleading sides of the reproduction element 55 to sandwich thereproduction element 55. The lower ends of the reproduction element 55and the upper and lower shields 56 and 57 are exposed to the ABS 13 ofthe slider 15. The reproduction head 54 is connected to the headamplifier IC 30 via an electrode (not shown), an interconnect (notshown) and an interconnect member 28 to output the read data to the headamplifier IC 30.

The recording head 58 is provided on the side of the trailing end 15 bof the slider 15 with respect to the reproduction head 54. The recordinghead 58 includes a main magnetic pole 60 made of high magneticpermeability material that generates a recording magnetic field in adirection perpendicular to the surface of the magnetic disk 12, a returnmagnetic pole 62 serving as a trailing shield (write shield, or firstshield), and a leading core 64 serving as a leading shield (secondshield). The main magnetic pole 60 and return magnetic pole 62constitute a first magnetic core that forms a magnetic path, and themain magnetic pole 60 and leading core 64 constitute a second magneticcore that forms a magnetic path. The recording head 58 includes a firstcoil (recording coil) 70 wound around the first magnetic core and asecond coil (recording coil) 72 wound around the second magnetic core.

The main magnetic pole 60 extends in a direction almost perpendicular tothe surface of the magnetic disk 12. The tip portion 60 a of the mainmagnetic pole 60 on the side of the magnetic disk 12 is tapered towardthe surface of the disk and its section is shaped like, for example, atrapezoid. The tip end face of the main magnetic pole 60 is exposed tothe ABS 13 of the slider 15. The width of a trailing side end face 60 bof the tip portion 60 a almost corresponds to that of a track in themagnetic disk 12.

The return magnetic pole 62 formed of a soft magnetic body is disposedon the trailing side of the main magnetic pole 60 in order to close themagnetic paths effectively via a soft magnetic layer 102 of the magneticdisk 12 immediately below the main magnetic pole 60. The return magneticpole 62 is approximately L-shaped and includes a first connectingportion 50 to he connected to the main magnetic pole 60. The firstconnecting portion 50 is connected to the upper portion of the mainmagnetic pole 60, or a portion separated from the ABS 13 of the mainmagnetic pole 60 via a non-conductive body 52.

The tip portion 62 a of the return magnetic pole 62 is shaped like anelongated rectangle and its tip end face is exposed to the ABS 13 of theslider 15. The leading side end face 62 b of the tip portion 62 aextends along the width direction of the tracks of the magnetic disk 12and also extends in a direction almost perpendicular to the ABS 13. Theleading side end face 62 b is opposite and almost parallel to thetrailing side end face 60 b of the main magnetic pole 60 with a writegap WG therebetween.

The first coil 70 is disposed to be wound around a magnetic circuit(first magnetic core) including the main magnetic pole 60 and the returnmagnetic pole 62. The first coil 70 is wound around, for example, thefirst connecting portion 50. When a signal is written to the magneticdisk 12, if recording current is caused to flow through the first coil70, the first coil 70 excites the main magnetic pole 60 to causemagnetic flux to flow through the main magnetic pole 60.

A spin torque oscillator (STO) 65, which is one example of thehigh-frequency oscillator, is provided between the tip portion 60 a ofthe main magnetic pole 60 and the return magnetic pole 62 in the writegap WG and its part is exposed to the ABS 13. The STO 65 is formed ofthree layers of a spin injection layer (Pin layer), an intermediatelayer and an oscillation layer and is so configured that when an STOvoltage is driven, the oscillation layer receives spin torque from thespin injection layer and is oscillated and magnetized.

The lower end face of the STO 65 is riot always flush with the ABS 13but can be separated upwardly in the height direction from the ABS 13.The spin injection layer, intermediate layer and oscillation layer canbe so formed that their laminate surfaces or film surfaces are inclinedrelative to a direction perpendicular to the ABS 13.

Connection terminals 91 and 92 are connected to the main magnetic pole60 and the return magnetic pole 62, respectively and also connected tothe head amplifier IC 30 via an interconnect. Thus, a current circuit isconfigured to allow current to flow in series from the head amplifier IC30 through the main magnetic pole 60, STO 65 and return magnetic pole62. Connection terminals 97 and 98 are connected to the recording heater19 a and the reproduction heater 19 b, respectively and also connectedto the head amplifier IC 30 via an interconnect.

The leading core 64 formed of a soft magnetic body is provided oppositeto the main magnetic pole 60 on the leading side of the main magneticpole 60. The leading core 64 is approximately L-shaped and its tipportion 64 a on the side of the magnetic disk 12 is shaped like anelongated rectangle. The tip end face (lower end face) of the tipportion 64 a is exposed to the ABS 13 of the slider 15. The trailingside end face 64 b of the tip portion 64 a extends along the widthdirection of tracks of the magnetic disk 12. The trailing side end face64 b is opposed to the leading side end face of the main magnetic pole60 with a gap therebetween. The gap is covered with a protectiveinsulating film 76 serving as a nonmagnetic body.

The leading core 64 includes a second connecting portion 68 joined to aback gap between the leading core 64 and the main magnetic pole 60 at aposition separated from the magnetic disk 12. The second connectingportion 68 is formed of, for example, a soft magnetic body and forms amagnetic circuit together with the main magnetic pole 60 and the leadingcore 64. The second coil 72 of the recording head 58 is disposed to bewound around a magnetic circuit (second magnetic core) including themain magnetic pole 60 and the leading core 64 to apply a magnetic fieldto the magnetic circuit. The second coil 72 is wound around, forexample, the second connecting portion 68. Note that a non-conductor ora nonmagnetic body can be inserted in part of the second connectingportion 68.

The second coil 72 is wound in a direction opposite to the direction inwhich the first coil 70 is wound. The first coil 70 and the second coil72 are connected to their respective terminals 95 and 96, and theseterminals 95 and 96 are connected to the head amplifier IC 30 via aninterconnect. The second coil 72 can be connected in series with thefirst coil 70. Furthermore, the first and second coils 70 and 72 mayeach control the supply of current. The currents supplied to the firstand second coils 70 and 72 are controlled by the head amplifier IC-30and the main controller 40.

The reproduction head 54 and recording head 58 are covered with theprotective insulating film 76, excluding a portion of the slider 15exposed to the ABS 13. The protective insulating film 76 forms an outershape of the head portion 17. In addition, a surface opposite to themagnetic disk 12 which includes the STO 65 is covered with a diamondlike carbon (DLC) protective film 101. In addition, one or moretemperature sensors 102 are provided inside the device (particularly,near the STO 65).

The head amplifier IC 30 that drives the magnetic head 16 and recordinghead 58 configured as described above includes, as shown in FIG. 1, arecording current supply circuit 81 that supplies recording current tothe first and second coils 70 and 72 via the connecting terminals 95 and96, an STO bias voltage application circuit 82 that applies an STO biasvoltage to the STO 65 via an interconnect (not shown) and the connectingterminals 91 and 92, a heater voltage application circuit 83 thatapplies a heater voltage to the recording heater 19 a and reproductionheater 19 b via an interconnect (not shown) and the connecting terminals97 and 98, and a measurement circuit 84 that measures and compares errorrates of data recorded on the magnetic disk 12. The head amplifier IC 30also includes a timing computation unit (not shown) which controls timeand timing at which current is caused to flow Through the recordingcurrent supply circuit 81 and also controls time and timing at which avoltage is applied to the STO bias voltage application circuit 82 and arecording current waveform generator (not shown) which generates arecording current waveform in response to a recording pattern signalgenerated from the R/W channel 42.

During the operation of the HDD 10, the main controller 40 drives thespindle motor 14 by the driver IC 48 and rotates the magnetic disk 12 ata given speed under control of the MPU 46. The main controller 40 drivesthe VCM 22 by the driver IC 48 to move the magnetic head 16 onto adesired track of the magnetic disk 12 and position it thereon.

During the recording, the recording current supply circuit 81 of thehead amplifier IC 30 causes recording current (AC) to pass through thefirst and second coils (referred to as recording coils hereinafter) 70and 72 in accordance with recording data and recording pattern generatedfrom the R/W channel 42. Thus, the first and second coils 70 and 72excite the main magnetic pole 60 to generate a recording magnetic fieldfrom the main magnetic pole 60. Under control of the MPU 46, the STObias voltage application circuit 82 applies an STO bias voltage to themain magnetic pole 60 and return magnetic pole 62 to energize theinterconnect, connecting terminals 91 and 92, main magnetic pole 60, STO65 and return magnetic pole 62 in series. Under control of the MPU 46,the heater voltage application circuit 83 and measurement circuit 84measure a recording data error rate and manages the temperature of therecording heater 19 a based upon a result of the measurement.

FIG. 3 is a block diagram showing a control system to control a biasvoltage when the STO is driven in the first embodiment, and FIG. 4 is aflowchart showing a control process of the control system. The controlsystem shown in FIG. 3 is implemented by a memory unit 300, apreamplification unit 400 and a system-on-chip (SoC) unit 500 in the MPU46. The memory unit 300 includes an STO bias voltage setting table 301that tables the relationship between a data sampling frequency and theoptimum STO bias voltage and an STO bias polarity setting table 302 thattables the relationship between a data sampling frequency and thepolarity of the optimum SIC bias voltage. The preamplification unit 400includes a write driver 401, an SIO bias voltage controller 402 and anSTO bias polarity controller 403. The SoC unit 500 includes a recordingdata generator 501. Upon receipt of a data write host command, therecording data generator 501 generates recording data for writing at adata sampling frequency designated by the command and also generatessetting information of the data sampling frequency.

More specifically, in the HDD according to the first embodiment, whendesired data. is recorded in response, to a processing command from ahost controller (not shown), the recording data generated by therecording data generator 501 in the SoC unit 500 is transmitted to thewrite driver 401 in the preamplification unit 400. The write driver 401calculates recording currents to record the recording data on themagnetic disk 12 and supplies the magnetic head 16 with a recordingcurrent corresponding to the recording current supply circuit 81 of thehead amplifier IC 30. Data is thus recorded on a recording layer of themagnetic disk 12.

During the foregoing data recording, the STO bias voltage is controlledas shown in FIG. 4. First, in the preamplification unit 400, datarecording is started (step S11). The preamplification unit 400 receivessetting information of a data sampling frequency to be generated whenrecording data is generated, from the SoC unit 500 and transfers it tothe STO bias voltage controller 402 and STO bias polarity controller 403(step S12). Upon receiving the setting information of a data samplingfrequency, the STO bias voltage controller 402 refers to the STO biasvoltage setting table 301 (step S13), reads out an STO bias voltagecorresponding to the sampling frequency of recording data that is beingwritten, and controls the STO bias voltage application circuit 82 tooutput the STO bias voltage (step S14). Similarly, upon receiving thesetting information of a data sampling frequency, the STO bias polaritycontroller 403 refers to the STO bias polarity setting table 302 (stepS15), reads out polarity of the STO bias voltage corresponding to thesampling frequency of recording data that is being written, and controlsthe STO bias voltage application circuit 82 to output the STO biasvoltage of the polarity (step S16). After that, a series of processes isrepeated until data recording is completed (step S17). Therefore, underthe condition of a high data sampling frequency at which an inhibitioninfluence is easily caused on an adjacent track due to the oscillationresponsiveness of the STO 65, the STO bias voltage can be optimized andaccordingly the inhibition influence can be reduced.

Below is one example of STO bias control in the first embodiment.

In FIG. 5, characteristics (a) to (c) each represent an example ofsetting an STO bias voltage to the data sampling frequency. As describedabove, an inhibition influence is caused due to oscillationresponsiveness with the increase of the data sampling frequency. It isthus desirable to suppress the STO bias voltage as the data samplingfrequency becomes higher. As a method for the suppression, for example,it is considered that the bias voltage is lowered uniquely relative tothe reference bias as the data sampling frequency increases, asrepresented as, e.g., characteristic (a) in FIG. 5.

On the other hand, it is considered that the above inhibition influencedue to oscillation responsiveness is not caused unless the frequency isa given frequency or higher. It is thus considered that in a frequencydomain of a predetermined frequency (threshold value Vth-b) or higher asrepresented as, e.g., characteristic (b) in FIG. 5, the bias voltage islowered uniquely relative to the reference bias in accordance with thefrequency. When the inhibition influence due to oscillationresponsiveness is great to the contrary, it may be desirable not toapply a bias voltage when the frequency is a given frequency (thresholdvalue Vth-c) or higher. It is thus considered that the bias voltage islowered uniquely relative to the reference bias in accordance with thefrequency in a frequency domain of threshold value Vth-c or lower asrepresented as characteristic (c) in FIG. 5 and the application of thebias voltage is turned off in a frequency domain of threshold valueVth-c or higher.

When a difference absolute value between an STO resistance value at thetime of application of a bias voltage to a polarity at which the STOoscillates and an STO resistance value at the time of application of abias voltage to a polarity opposite to that in the oscillation directionis delta R as shown in, e.g., FIG. 6A, the reference bias can beconfirmed by measuring the difference absolute value delta R with theincrease of the absolute value of the bias voltage. For example, asshown in FIG. 6A, the bias voltage value obtained when the differenceabsolute value delta R has reached 90% of the maximum value thereof, hasonly to be defined as a reference bias. Alternatively, as shown in FIG.6B, when a difference absolute value between an overwrite characteristicat the time of application of a bias voltage to a polarity at which theSTO oscillates and an overwrite characteristic at the time ofapplication of a bias voltage to a polarity opposite to that in theoscillation direction is defined as delta OW, the delta OW is measuredwith the increase of the absolute value of the bias voltage and, forexample, the bias voltage obtained when the delta OW has reached 90% ofthe maximum value thereof, has only to be defined as a reference bias.

A specific assumed range of the foregoing frequency threshold valuesVth-b and Vth-c is inferred from the results of recording densityimprovement rate with respect to the data sampling frequency at the timeof application of a predetermined STO bias voltage, as shown in FIG. 7.The inhibition influence on an adjacent track due to responsiveness ofthe STO tends to deteriorate the recording density improvement rate asthe data sampling frequency becomes higher. It is seen that the samplingfrequency at which the improvement rate is less than 0% and therecording density is lowered, is approximately 1880 MHz or higher and3230 MHz or lower within a f1 to f2 range of the average value ±1σ(characteristic is an average value, characteristic (b) is −1σ andcharacteristic (c) is +1σ). This frequency has only to be the foregoingfrequency threshold values Vth-b and Vth-c.

As described above, the STO bias voltage is adjusted according to thedata sampling frequency to make it possible to reduce the inhibitioninfluence due to responsiveness of the STO. It is effective to adjustthe STO bias voltage to suppress the peak value of the recording currentaccording to the data sampling frequency and lengthen the rise time ofthe recording

Below is a description of advantages to be produced from the firstembodiment. FIG. 8 shows time variations of the width of interferencewith an adjacent track (erase width) at the time of recording of data of2.5 GHz data sampling frequency. In FIG. 8, the solid line indicates thefirst embodiment and the dotted line indicates the prior art. When afixed bias voltage is applied irrespective of the data samplingfrequency as indicated by the dotted line (prior art), an electric fieldleaks widely from the main magnetic pole due to the influence of a delayof STO oscillation responsiveness to a recording main magnetic poleimmediately after the polarity of recording data is reversed. At thatmoment, the erase width greatly increases to deteriorate the quality ofadjacent data. In the first embodiment, when a bias voltage is loweredaccording to the frequency to suppress the influence of the STOoscillation as indicated by the solid line, the erase width can beprevented from increasing extremely even immediately after the polarityof recording data is reversed as shown in FIG. 8.

FIG. 9 shows results of measurement of recording density that isattainable when the data sampling frequency is increased. In FIG. 9, Aindicates that the first embodiment is not applied (prior art) and B hindicates that the first embodiment is applied. In the prior artrepresented as characteristic A, the inhibition influence due tooscillation responsiveness is small and not problematic in a low datasampling frequency domain, but the recording density capable of readingand writing will be lowered by the inhibition influence due tooscillation responsiveness in a high data sampling frequency domain. Incontrast, if the first embodiment is applied, the bias voltage islowered in accordance with the data sampling frequency to reduce theinfluence of SIC oscillation, with the result that the decrease inrecording density due to the increase in frequency can be mitigated asrepresented as characteristic B in FIG. 9, thus improving in recordingdensity.

As described above, in the magnetic disk device according to the firstembodiment, in a high data sampling frequency domain where theinhibition influence on an adjacent track due to SIC oscillationresponsiveness is easy to occur, the setting of the STO bias voltage canbe optimized and thus the inhibition influence can be reduced.

Second Embodiment

In the first embodiment, a bias voltage is controlled in accordance withthe sampling frequency of recording data. The recording data samplingfrequency is adjusted based upon the radial position of a magnetic disk.It is thus understood that the control of a bias voltage at a samplingfrequency of recording data is equivalent to the control of a biasvoltage in the radial position of a magnetic disk for writing. In thesecond embodiment, a bias voltage is controlled in accordance with theradial position of a magnetic disk for writing. This bias voltagecontrol will be described.

In the bias control according to the second embodiment, as shown in FIG.3, the SoC unit 500 transfers the setting information of recording datawrite position (the radial position of a magnetic disk) to the STO biasvoltage controller 402 and the STO bias polarity controller 403. The STObias voltage controller 402 controls a bias voltage to be applied to theSTO 65 in the magnetic head 16, based upon the bias voltage settingtable 301 stored in the memory unit 300 in accordance with the settinginformation of the transferred disk radial position. Similarly, the STObias polarity controller 403 controls the polarity of a bias voltage tobe applied to the STO 65 in the magnetic head 16, based upon the biaspolarity setting table 302 stored in the memory unit 300 in accordancewith the setting information of the transferred disk radial position.

During the foregoing data recording, the STO bias voltage is controlledas shown in FIG. 10. First, in the preamplification unit 400, datarecording is started (step S21). The preamplification unit 400 receiveswrite disk radial position setting information to be generated whenrecording data is generated, from the SoC unit 500 and transfers it tothe STO bias voltage controller 402 and STO bias polarity controller 403(step S22). Upon receiving the disk radial position setting information,the STO bias voltage controller 402 refers to the STO bias voltagesetting table 301 (step S23), reads out an STO bias voltagecorresponding to the disk radial position of recording data that isbeing written, and controls the STO bias voltage application circuit 82to output the STO bias voltage (step S24). Similarly, upon receiving thedisk radial position setting information, the SIC bias polaritycontroller 403 refers to the STO bias polarity setting table 302 (stepS25), reads out polarity of the STO bias voltage corresponding to thedisk radial position of recording data that is being written, andcontrols the STO bias voltage application circuit 82 to output the STObias voltage of the polarity (step S26). After that, a series ofprocesses is repeated until data recording is completed (step S27).Therefore, under the condition of a high data sampling frequency atwhich an inhibition influence is easily caused on an adjacent track dueto the oscillation responsiveness of the STO 65, the SIC bias voltagecan be optimized and accordingly the inhibition influence can hereduced.

Below is one example of STO bias control in the second embodiment.

In FIG. 11, characteristics (a) to (c) each represent an example ofsetting an STO bias voltage to the disk radial position. As describedabove, an inhibition influence is caused due to oscillationresponsiveness as the disk radial position is made closer to the outercircumference. It is thus desirable to suppress the STO bias voltage asthe disk radial position is made closer to the outer circumference. As amethod for the suppression, for example, it is considered that the biasvoltage is lowered uniquely relative to the reference bias as the diskradial position is made closer to the outer circumference, asrepresented as, e.g., characteristic (a) in FIG. 11.

On the other hand, it is considered that the above inhibition influencedue to oscillation responsiveness is not caused unless the disk radialposition is beyond a given one. It is thus considered that in afrequency domain of a predetermined frequency (threshold value Rth-b) orhigher as represented as, e.g., characteristic (b) in FIG. 11, the biasvoltage is lowered uniquely relative to the reference bias in accordancewith the frequency. When the inhibition influence due to oscillationresponsiveness is great to the contrary, it may be desirable not toapply a bias voltage when the frequency is a given frequency (thresholdvalue Rth-c) or higher. It is thus considered that the bias voltage islowered uniquely relative to the reference bias in accordance with thefrequency in a disk radial domain of threshold value Rth-c or lower asrepresented as characteristic (c) in FIG. 11 and the application of thebias voltage is turned off in a disk radial domain of threshold valueRth-c or higher.

As in the first embodiment, the reference bias is defined as a biasvoltage value obtained when, for example, the difference absolute valuedelta R has reached 90% of the maximum value thereof or a bias voltagevalue obtained when, for example, the delta OW has reached 90% of themaximum value thereof.

A specific assumed range of the foregoing frequency threshold valuesRth-b and Rth-c is inferred from the results of recording densityimprovement rate with respect to the disk radial position at the time ofapplication of a predetermined STO bias voltage. The inhibitioninfluence on an adjacent track due to responsiveness of the SAO tends todeteriorate the recording density improvement rate as the disk radialposition is made closer to the outside. Considering a disk radialposition at which the improvement rate is less than 0% and the recordingdensity is lowered, the voltage switching position has only to fallwithin an area with a disk radius of 27 mm or larger and 54 mm orsmaller. This radius position is thus defined as the foregoing thresholdvalues Rth-b and Rth-c.

As described above, the STO bias voltage is adjusted according to thewrite disk radial position to make it possible to reduce the inhibitioninfluence due to responsiveness of the STO. It is effective to adjustthe STO bias voltage to suppress the peak value of the recording currentaccording to the disk radial position and lengthen the rise time of therecording current according to the disk radial position.

As described above, in the magnetic disk device according to the secondembodiment, in the outer circumference of the disk radial position wherethe inhibition influence on an adjacent track due to STO oscillationresponsiveness is easy to occur, the setting of the STO bias voltage canbe optimized and thus the inhibition influence can be reduced.

Third Embodiment

In the first and second embodiments, the SIC bias voltage is controlledbased upon a data sampling frequency or a disk radial positioncorresponding to the data sampling frequency. Furthermore, theinhibition influence due to oscillation responsiveness of the STO can beimproved even by controlling the polarity of the STO bias voltage by theSTO bias polarity setting table 302 and SIC bias polarity controller 403shown in FIG. 3. In other words, when the data sampling frequency is apolarity switching threshold value or higher, the energizing polarity ofthe STO bias is switched to the polarity opposite to the direction inwhich the STO oscillates. This situation is shown in FIG. 12. Thepolarity switching threshold value can be set to 1880 MHz or larger and3230 MHz or smaller.

The polarity of the STO bias voltage can be switched in accordance withthe radial position of a disk on which data is recorded. When the diskradial position is outside the polarity switching position, theenergizing polarity of the STO bias is switched to the polarity oppositeto a direction in which the STO oscillates. This situation is shown inFIG. 13. The polarity switching position can be set to an area with adisk radius of 27 mm or larger and 54 mm or smaller.

Fourth Embodiment

The fourth embodiment is directed to a technique of optimizing recordingcurrent when an STO bias voltage is adjusted in order to preventadjacent track data quality deterioration due to STO oscillationresponsiveness further at the time of recording of data with a high datasampling frequency. FIG. 14A shows recording current dependency on biterror rate at the time of application of a reference bias voltage. Therecording current (reference recording current) at the time of recordingdata is set based upon the condition that a bit error rate has reached athreshold value when the threshold value is, for example, 98% of theabsolute value thereof. On the other hand, when a bias voltage islowered from the reference bias voltage in accordance with the datasampling frequency as shown in FIG. 14B, the rising of a bit error ratebecomes late as the recording current increases. This causes the problemof deteriorating on-track properties slightly with the set referencerecording current unchanged. To lower the bias voltage, it is desirableto confirm a recording current value that reaches a threshold value andset it higher than the reference recording current when necessary. Tolower the bias voltage relative to the reference bias in accordance withthe data sampling frequency and reduce the influence of STO oscillation,the recording current is also adjusted to he higher than the referencerecording current. It is thus possible to prevent the interference withan adjacent track and maintain the on-track quality as much as possible.

In the foregoing description, the bias voltage is controlled inaccordance with the data sampling frequency to adjust the recordingcurrent. The same holds true for the case where the bias voltage iscontrolled in accordance with the write disk radial position.

As described above, the magnetic disk device according to the fourthembodiment makes it possible to prevent an adjacent track signal qualitydeterioration due to STO oscillation responsiveness and improve thesignal quality and the recording density.

Fifth Embodiment

The present embodiment relates to a manner of optimizing the biasvoltage applied to STO in accordance with the circumferential speed ofthe disk and the data sampling frequency, to avoid deterioration of thereliability of the STO, i.e., the oscillation characteristic of the STOwhen operated for a long time. When the bias voltage is applied to theSTO for a long time, a diamond like carbon (DLC) protective film on ahead floating surface is deteriorated together with a local rise intemperature caused by application of the bias voltage, as the problem ofreliability.

One of methods of verifying the variation in quality of the DLCprotective film is Raman spectroscopic analysis. In general, the qualityof the DLC protective film can be measured based on the spectralintensity of D band (up to 1350 cm⁻¹) caused by extension of asix-membered ring and a ratio ID/IG of the G peak intensity resultingfrom sp2 binding oscillation. The six-membered ring in the filmincreases as ID/IG becomes larger. This means that graphitecrystallization proceeds and hardness of the

DLC protective film is degraded.

In fact, a result of measuring the Raman spectroscopic intensity ratioID/IG after baking at each temperature for 10 hours to observe thetemperature resistance of the DLC protective film is shown in FIG. 15.In FIG. 15, Error bar indicates the maximum value and the minimum valueat each temperature. Variation in the peak intensity ratio is notobserved even when the baking temperature is raised up to 140° C., butif the temperature is further raised, ID/IG rises and a situation ofdeterioration of the DIG protective film can be confirmed. When thetemperature condition that the maximum value or minimum value of ID/IGis 0.6 or more of the ID/IG ratio is considered, compensated temperatureId of DLC is considered to be in a range from 150 to 180° C.

As described above, the bias voltage of the STO needs to be adjusted asneeded to set the local temperature near the STO to he in a range whichdoes not exceed the temperature, to avoid head disk interface (HDI)failure caused by deterioration of the DLC protective film in an actualdrive operation. In addition, since the temperature near the STO dependson the air cooling effect resulting from disk rotation, its influenceneeds to be considered.

FIG. 16 shows a result of calculating the rising temperature of the STOwhile varying the circumferential speed of the disk rotation under astatic condition in an unloaded state, and under a dynamic condition ina state of being loaded on the disk, when the bias voltage is applied tothe STO in an environment in which the internal temperature of themagnetic disk device is 60° C. The applied bias voltage is set as avoltage having a current density equivalent to 4×10̂8 A/cm² at whichmagnetization of the oscillation layer of the STO reaches in-plane 90degrees in a macro spin Model.

As understood from this result, the temperature near the STO is lowerthan or equal to a threshold value Td of the DLC compensated temperatureby the air cooling effect of the disk under the condition that thecircumferential speed is high but, if the circumferential speed is lowerthan or equal to circumferential speed Vd, the temperature near the STOrises up to a temperature above the threshold value Td due to reductionin the air cooling effect, and a risk of deterioration of the DLCprotective film is generated. Therefore, when the circumferential speedis lower than or equal to the circumferential speed Vd, the bias voltageneeds to be lower than the condition of an ideal oscillation intensity.Since the threshold value Td of the compensated temperature is in arange from 150 to 180° C. as described above, the circumferential speedVd is desirably 17.4 m/s or more and 45.3 m/s or less, based on theresult of FIG. 16.

FIG. 17 shows an example of controlling the bias voltage in accordancewith the circumferential speed so as to prevent the temperature near theSTO from exceeding the threshold value Td. In FIG. 17, it is consideredthat the bias voltage is uniformly controlled to be suppressed as thecircumferential speed becomes lower than predetermined circumferentialspeed Vd, as represented by A, and is also considered that the biasvoltage is controlled not to be applied when the circumferential speedis lower than the predetermined circumferential speed Vd, as representedby B. In addition, in general, since the circumferential speed in themagnetic disk device, the data sampling frequency, and the position inthe radial direction (radial position) have a unique relationship, thebias voltage may be controlled to be suppressed when the data samplingfrequency is lower than or equal to predetermined threshold value fd, asshown in FIG. 18. In addition, as shown in FIG. 19, the bias voltage maybe controlled to be suppressed in an area in which the recordingposition of disk is inner than a predetermined radial position rd.Desirably, the frequency fd is in a range of 1620 MHz or higher and 3720MHz or lower and the radial position rd is in a range of 23.1 mm orhigher and 60 mm or lower, based on the desired ranges of thecompensated temperature Td and circumferential speed Vd.

As described above, the deterioration of the DLC protective film basedon a write time can be prevented and the rise of the hit error rate canbe suppressed by performing control to reduce the STO bias voltage basedon the data sampling frequency, circumferential speed, and radialposition. This situation is shown in FIG. 20. In FIG. 20, A represents acase where the STO bias voltage is not controlled, while B represents acase where the SIC bias voltage is controlled to be lowered by applyingthe present embodiment.

Sixth Embodiment

As described above, since the local temperature near STO needs to be setto be lower than the DLC compensated temperature to preventdeterioration of the DLC protective film of the recording head from theviewpoint of reliability, the adjustment of the STO bias voltageaccording to the temperature inside the magnetic disk device is alsoimportant.

FIG. 21 shows a result of calculating a temperature of the STO whenapplying the bias voltage to the STO relative to a device internaltemperature at each portion, in an innermost circumferential area of adisk. The applied bias voltage is set as a voltage having a currentdensity equivalent to 4×10̂8 A/cm² at which magnetization of the STOoscillation layer reaches in-plane 90 degrees in a macro spin model. Inan environment of a certain bias voltage, the temperature near the STObecomes higher as the device internal temperature becomes higher, and ifthe temperature becomes certain device internal temperature ATd thetemperature exceeds compensated temperature Td of the DLC protectivefilm. For this reason, the bias voltage needs to be controlled to belowered in an innermost area of the disk. As described above, the deviceinternal temperature Atd is desirably 10° C. or higher and 40° C. orlower, based on the relationship shown in FIG. 21, since the thresholdvalue Id of the compensated temperature is in a range from 150 to 180°C.

FIG. 22 shows an example of controlling the bias voltage in accordancewith a temperature inside the device so as to prevent a temperature nearthe STO from exceeding a threshold value Td of a compensatedtemperature. In FIG. 22, it is considered that the bias voltage isuniformly controlled to be suppressed as the internal temperature of thedevice becomes higher than predetermined temperature ATd, as presentedby A, and is also considered that the bias voltage is controlled not tobe applied in a temperature range where the temperature is higher thanthe predetermined temperature ATd as presented by B. The temperatureinside the device is measured by, for example, a temperature sensordisposed inside the device.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied is a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fail within the scope andspirit of the inventions.

What is claimed is:
 1. A magnetic disk device, comprising: a magneticdisk including a recording layer; a recording head configured to apply arecording magnetic field to the recording layer by a main magnetic poleand a return magnetic pole; and a controller configured to control therecording head, wherein the recording head includes: a high-frequencyoscillator disposed in a write gap between the main magnetic pole andthe return magnetic pole and configured to oscillate in accordance witha bias voltage; and a bias voltage supply circuit configured to supplythe bias voltage to the high-frequency oscillator, and the controllerincludes a bias voltage controller configured to control the biasvoltage to he applied to the high-frequency oscillator in accordancewith a sampling frequency of data when the data is recorded on themagnetic disk by the recording head.
 2. The magnetic disk device ofclaim 1, wherein the recording head comprises a protective film coveringa surface opposite to the magnetic disk including the high-frequencyoscillator, and the bias voltage controller controls the bias voltage tolower relative to a reference bias as the sampling frequency becomeslower.
 3. The magnetic disk device of claim 2, wherein the bias voltagecontroller controls the bias voltage to lower relative to a referencebias or turn off when the sampling frequency is lower than or equal to athreshold value of voltage switching.
 4. The magnetic disk device ofclaim 3, wherein the bias voltage controller sets the threshold value ofthe voltage switching to 1620 MHz or higher and 3720 MHz or lower.
 5. Amagnetic disk device, comprising: a magnetic disk including a recordinglayer; a recording head configured to apply a recording magnetic fieldto the recording layer by a main magnetic pole and a return magneticpole; and a controller configured to control the recording head, whereinthe recording head includes: a high-frequency oscillator disposed in awrite gap between the main magnetic pole and the return magnetic poleand configured to oscillate in accordance with a bias voltage; and abias voltage supply circuit configured to supply the bias voltage to thehigh-frequency oscillator, and the controller includes a bias voltagecontroller configured to control the bias voltage in accordance with adata recording position in a radial direction of the magnetic disk whenthe data is recorded on the magnetic disk by the recording head.
 6. Themagnetic disk device of claim 5, wherein the recording head comprises aprotective film covering a surface opposite to the magnetic diskincluding the high-frequency oscillator, and the bias voltage controllercontrols the bias voltage to lower relative to a reference bias as thedata recording position of the magnetic disk becomes an innercircumferential area in a radial direction.
 7. The magnetic disk deviceof claim 6, wherein the bias voltage controller controls the biasvoltage to lower relative to a reference bias or turn off when the datarecording position of the magnetic disk is on an inner circumferentialside than a threshold value of voltage switching in a radial direction.8. The magnetic disk device of claim 7, wherein the bias voltagecontroller sets the voltage switching position to fall within an areacorresponding to a radius of 23.1 mm or larger and 60 mm or smaller ofthe magnetic disk.
 9. A magnetic disk device, comprising: a magneticdisk including a recording laver; a recording head configured to apply arecording magnetic field to the recording layer by a main magnetic poleand a return magnetic pole; and a controller configured to control therecording head, wherein the recording head includes: a high-frequencyoscillator disposed in a write gap between the main magnetic pole andthe return magnetic. pole and configured to oscillate in accordance witha bias voltage; and a bias voltage supply circuit configured to supplythe bias voltage to the high-frequency oscillator, and the controllerincludes a bias voltage controller configured to control the biasvoltage in accordance with a rotation circumferential speed of themagnetic disk when toe data is recorded on the magnetic disk by therecording head.
 10. The magnetic disk device of claim 9, wherein therecording head comprises a protective film covering a surface oppositeto the magnetic disk including the high-frequency oscillator, and thebias voltage controller controls the bias voltage to lower relative to areference bias as a rotation circumferential speed of the magnetic diskrecording the data becomes lower.
 11. The magnetic disk device of claim10, wherein the bias voltage controller controls the bias voltage tolower relative to a reference bias or turn off when the rotationcircumferential speed of the magnetic disk is lower than or equal to athreshold value of a voltage switching speed.
 12. The magnetic diskdevice of claim 11, wherein the bias voltage controller sets thethreshold value of the voltage switching speed to 17.4 m/s or higher and45.3 m/s or lower.
 13. A magnetic disk device, comprising: a magneticdisk including a recording layer; a recording head configured to apply arecording magnetic field to the recording layer by a main magnetic poleand a return magnetic pole; and a controller configured to control therecording head, wherein the recording head includes: a high-frequencyoscillator disposed in a write gap between the main magnetic pole andthe return magnetic pole configured to oscillate in accordance with abias voltage; and a bias voltage supply circuit configured to supply thebias voltage to the high-frequency oscillator, and the controllerincludes a bias voltage controller configured to control the biasvoltage in accordance with a casing internal temperature of the magneticdisk device when the data is recorded on the magnetic disk by therecording head.
 14. The magnetic disk device of claim 13, furthercomprising: a temperature sensor measuring a casing internaltemperature, wherein the recording head comprises a protective filmcovering a surface opposite to the magnetic disk including thehigh-frequency oscillator, and the bias voltage controller controls thebias voltage to lower relative to a reference bias as the measuredtemperature of the temperature sensor becomes higher.
 15. The magneticdisk device of claim 14, wherein the bias voltage controller controlsthe bias voltage to lower relative to a reference bias or turn off whenthe measured temperature of the temperature sensor is higher than orequal to a threshold value of a voltage switching temperature.
 16. Themagnetic disk device of claim 15, wherein the bias voltage controllersets the threshold value of the voltage switching temperature to 10° C.or higher and 40° C. or lower.